Janus chiral bound states in the continuum with maximum circular dichroism in photonic crystal heterostructures

Extremely high-quality-factor chiral metasurfaces hold great promise for applications in lasing, nonlinear optics, and nanophotonics. Here, we propose a novel, to the best of our knowledge, quasi-two-dimensional (q-2D) double-layered photonic crystal heterostructure (PhCH) that supports Janus chiral bound states in the continuum (BICs) with near-infinite quality factor (Q factor) and near-unity circular dichroism. By employing distinct dielectric layers and breaking the out-of-plane symmetry, Janus BICs with asymmetric topological charges are produced. In addition, precise tuning of in-plane mirror symmetry via lateral displacement of rectangular holes enables control over polarization singularities, yielding true Janus chiral BICs at the Γ point. In contrast to more intricate three-dimensional dielectric metasurfaces, our q-2D architecture requires only a single lithography step, greatly simplifying fabrication while preserving robust topological stability and exceptional chirality. Our findings open new avenues for the practical realization of high-Q chiral optical platforms in advanced photonic devices.